In order to convert an optical signal into an electrical signal at high speed and with high efficiency, photodiodes using a photoelectric conversion phenomenon in a semiconductor are widely used. The photodiodes are used, for example, in the field of communication and the field of information processing, for the purpose of converting an optical signal including infrared light, visible light, and ultraviolet light into an electrical signal. The photodiodes are classified into a pn type, a pin type, a Schottky barrier type, an MSM (metal-semiconductor-metal) type, and so forth on the basis of their structure. The principal factors that limit the response speed of the photoelectric conversion of a photodiode are: a circuit time constant that is determined by a product of load resistance connected to the photodiode and electrical capacitance created by a depletion layer in the photodiode; and a carrier transit time needed for carriers to pass through the depletion layer. Thus, in order to improve fast response of the photodiode, either the circuit time constant should be reduced or the carrier transit time should be shortened.
One of photodiodes capable of fast response includes an MSM type, and is expected as a photoelectric conversion device used in the fields of communication and information processing. The MSM photodiode is a type of Schottky barrier photodiode. In the MSM photodiode, a pair of electrodes are disposed on a surface of a semiconductor layer functioning as a light absorption layer, and a Schottky barrier is formed in the vicinity of each of the two electrodes. Japanese Patent Application Laid-Open No. 7-153989 (JP-A-7-153989), which corresponds to EP 0651448 A1, discloses an example of a typical MSM photodiode. In the MSM photodiode, by using two electrodes each having a comb-like structure and arranging these electrodes in an interdigitated manner, a high electric field can be applied to the light absorbing layer even if a voltage applied to the photodiode is low and the carrier transit time can be thus shortened, thereby achieving a relatively fast response speed. On the other hand, there is a problem that because the incident light is reflected by the electrodes disposed on a light receiving surface, the quantum efficiency decreases. In addition, there exists a trade-off relationship where, if the carrier transit time is shortened by thinning the light absorbing layer in order to attain fast response, this causes a drop in efficiency.
In recent years, various attempts have been made to further increase the operation speed of the MSM photodiode and further increase the quantum efficiency thereof through the use of a metal surface plasma phenomenon.
For example, Japanese Patent Application Laid-Open No. 10-509806 (JP-A-10-509806), which corresponds to WO96/05536, discloses a photoelectric coupler in which the surface plasmon phenomenon is used. In this photoelectric coupler, a device configuration is employed in which interdigital metal electrodes aligned with regular spacing are arranged on a semiconductor substrate such that positive electrodes and negative electrodes confront each other with one fitting into the other. In addition, JP-A-10-509806 (WO96/05536) describes an MSM photodiode in which incident light and surface plasmon are coupled with each other by resonance, and also describes that a diffraction wave generated by a metal electrode is coupled with a local wave and confined in a waveguide. However, this literature does not describe a structure of a metal electrode which efficiently generates a diffracted light by surface plasmon resonance. Moreover, as for a method for coupling a diffracted light to a waveguide formed by a light absorbing layer, the wave number matching condition alone is disclosed, but neither a waveguide structure for improving coupling efficiency nor a positional relationship with the metal electrode has been described. Therefore, in the MSM photodiode disclosed in JP-A-10-509806 (WO96/05536), the efficiency of generating a diffracted light of a desired order is low, and the coupling efficiency of the diffracted light to the waveguide formed by the light absorbing layer is low, thereby lowering the quantum efficiency.
Japanese Patent Application Laid-Open No. 2006-501638 (JP-A-2006-501638) corresponding to WO2004/012275 discloses an MSM photodiode which has a cavity structure formed by installing a Bragg reflection mirror below a light absorbing layer and using an electrode as a top mirror, and can improve efficiency even if the thickness of a light absorbing layer is small by confining a zero-order transmitted light within this cavity structure. However, in this MSM photodiode, the Q value of the cavity needs to be increased in order to achieve sufficient efficiency, but it is inherently difficult to make light incident into the cavity having a high Q value, and the incident light is reflected by the electrode functioning as the top mirror of the cavity before it enters the cavity. As a result, with this configuration, it is difficult to achieve sufficient quantum efficiency.
Japanese Patent Application Laid-Open No. 9-023022 (JP-A-9-023022) discloses a photoelectric conversion device which has an optical waveguide formed by sandwiching a multiple-quantum well layer as a core layer between upper and lower cladding layers, and allows the multiple-quantum well layer to absorb light. However, JP-A-9-023022 does not describe the improvement of the efficiency of an MSM photodiode using a diffracted light.